JIM DENT CONSTRUCTION
Project Overview
The Upper Ramona Tunnel Plug was the final stage of a hydroelectric generating project where a 6m high tunnel through a mountain intercepted a 500‐meter‐high shaft located in the Narrow Inlet. The shaft would later be flooded by draining a lake into it, creating 500 meters of head pressure to generate electricity. Multicrete Systems Incorporated (MSI) was tasked with installing a temperature sensitive shotcrete/cast‐in‐place tunnel plug measuring 25 meters long that held back the water pressure with a seepage criterion of 25 L/s. The tunnel plug encapsulated a 1‐meter diameter pipe through which water would be directed to the generating station.
Project Scope
Site Conditions
The first task was designing modular equipment that could be transported up the single lane mountain trail to the portal location. From there, MSI had to fabricate low profile equipment with width restrictions that could be transported up the 800‐meter‐long tunnel at a positive 10% grade.
The ambient air temperature outside was between +10 and ‐10 degrees Celsius, whereas the temperature at the pour location 800 meters up the tunnel stayed at a constant +10 degrees Celsius. In order to meet mining and Work Place Health and Safety regulations, the equipment to mix and pump the concrete needed to allow for access and egress of emergency personnel into the pour location.
The next challenge was achieving high volume pour rates using equipment limited by the dimensions of the tunnel. MSI used a pan style concrete mixer to mix cast in place concrete. The pan mixer was mounted on a custom‐made steel frame which helped achieve the height needed to discharge the concrete into a concrete line pump below it. The pan mixer was completely automated with two chemical dosing units used to dose air entrainment and super plasticizer to the concrete mixes.
Lastly, the hydrating concrete could not exceed +50 degrees Celsius anywhere throughout the mass of concrete or the structure would risk temperature cracking causing unwanted fissures for water to travel through. Temperature sensors were placed in strategic locations to monitor temperatures before, during and after the concrete pours to adhere to this specification. MSI chose to place the mass pour in three separate applications to allow the concrete enough time to cool down before applying the next mass of concrete.
Shotcrete
After the first two 2‐meter‐high lifts of cast in place concrete were poured, shotcrete was applied for the entire 25‐meter tunnel plug to seal off the tunnel. In order to prevent shearing between lifts of concrete, depressions were cast into each pour to create shear keys. Additionally, rubber water stops were installed between each lift of concrete to prevent water seepage between cold joints. Water stops and shear keys were only necessary between cast in place lifts due to the inherent nature of concrete but were not required in shotcrete as there are no cold joints (Figure 4).
Blow Pipes were used while shotcreting to remove shotcrete rebound from the walking surface to ensure a perfect water stopping seal (Figure 6). Additionally, the SikaFuko system was installed on the top and bottom portions of the tunnel plug in order to post grout the tunnel profile thus sealing all cracks in the surrounding rock surface (Figure 7).
Final Product
Once the entire tunnel was plugged and post grouted, the shaft above was flooded. The final seepage through the tunnel was measured at 5 L/s. This demonstrates after all the preparation and precautions taken before application, the seepage past the tunnel plug was 80% less than expected. Another successful project.
